1. Field of the Invention
The present invention relates to an electrophotographic apparatus, and more particularly to such an apparatus capable of regulating the image density.
2. Description of the Prior Art
In a microfilming apparatus or a microfilm reader-printer, the original image stored on a microfilm may be positive or negative. In either case the copy to be prepared from such original should be positive. In order to meet such requirement there is already known an electrophotographic apparatus which can be switched between a mode for forming a positive image from a negative original image (N-P mode) and another mode for forming a positive image from a positive original image (P-P mode).
In the following there will be explained the image formation in the N-P mode and in the P-P mode, in an electrophotographic apparatus shown in FIG. 1. In case of N-P mode, a drum-shaped photosensitive or image bearing member 1, having a photoconductive layer on a conductive member, is at first negatively charged by a primary charger 2 in a dark place and is exposed to a light from a negative original to form a negative electrostatic latent image. Said electrostatic latent image is subjected to a reversal development with negatively charged toner supplied from a developing unit 4. A bias voltage HAC composed of an AC voltage superposed with a negative DC voltage is applied to a developing sleeve 4a and a blade 4b whereby the negatively charged toner jumps to an exposed light area of a surface potential of about 0V thus achieving image development. The image composed of the negatively charged toner is then transferred onto a transfer sheet P by the application of a positive corona discharge by a transfer charger 5 from the rear side of said transfer sheet P. In the image formation in P-P mode, there is employed a developing unit 4 capable of supplying positively charged toner. The photosensitive member 1 negatively charged is exposed to the light 3 of a positive original to form a positive electrostatic latent image, which is directly developed with the positively charged toner. Said toner supplied from the developing unit 4 receiving the bias voltage HAC is deposited in the unexposed dark area of a negative surface potential on the photosensitive member 1. The image composed of the positively charged toner is transferred onto the transfer sheet P with a negative corona discharge of the transfer charger 5. Both in the N-P and P-P modes, a hard copy is obtained by fixing the image on the transfer sheet P. On the other hand, after image transfer, the toner remaining on the photosensitive member 1 is cleaned by a cleaning unit 6 and the retentive charge is dissipated by a uniform illumination 7. In this manner a next image forming cycle can be initiated. There are also shown a photoelectric sensor 8 for detecting the transfer sheet P, a slit 9 and a shutter 10.
In such an electrophotographic apparatus, the density of the obtained copy image is regulated by controlling the DC component, which will hereinafter be called developing bias voltage, of the biased AC voltage HAC applied to the developing sleeve 4a. As will be understood from FIG. 2, a higher developing bias voltage provides a higher density in the N-P mode but a lower density in the P-P mode. Conventionally, the developing bias voltage is regulated by a variable resistor linked with a density control knob, and an erroneous operation is apt to occur since the direction of control is inverted in the N-P and P- modes. Although there may be employed separate density control knobs and variable resistors respectively for the N-P and P-P modes, such method requires complicated operation because of the increased operating parts and does not necessarily prevent the error in operation. It may also result in an increased cost because of an increased number of parts.
In addition, the regulating range of the developing bias voltage for obtaining an adequate image density is not the same in both modes. Consequently, a single variable resistor, if employed for regulating the developing bias voltage, will provide the same regulating range for both modes and will therefore be unable to cover the optimum image density ranges in both modes.
Furthermore, in the N-P mode the operating positive voltage of the charger 5 is made higher for improving the efficiency of image transfer, and such higher positive voltage shifts the surface potential of the photosensitive member 1 to positive, contrary to the charging characteristic thereof. Consequently, the retentive charge cannot be sufficiently eliminated unless the illumination for charge elimination is made considerably strong. Particularly in a space between the transfer sheets P, positive corona ions directly reach the photosensitive member 1 to generate a higher potential than in an area subjected to the corona discharge through the transfer sheet P. Experimentally a voltage of the transfer charger, that will generate a potential of +80V, by corona discharge through the transfer sheet, on the photosensitive member showing a surface potential of -800V after primary charging, generates a potential of +500V by direct corona discharge without the transfer sheet. The retentive charge of such magnitude gives rise to an uneven charge elimination even if the illumination 7 is made strong. A primary charging in the next imaging cycle, if applied after such uneven charge elimination, will result in uneven surface potential. The peripheral length of the photosensitive member 1 is often shorter than the length of the transfer sheet P, so that a copying cycle often requires two or several turns of the photosensitive member 1. In the second and ensuing turns the photosensitive member 1 has already been subjected to the transfer corona discharge, so that the image formation in a copy is conducted with different states of primary charging, thus resulting in uneven image density.
On the other hand, in the P-P mode, a nonimage area not subjected to imagewise exposure is developed black because of the presence of a charge, thus giving an unpleasant black frame adjacent to image area, and also wasting the toner. In order to avoid such unnecessary development, a uniform illumination, called blank exposure, is conventionally given to the non-image area, but such blank exposure tends to generate a background smudge or a black streak at the beginning or at the end of image exposure on the photosensitive member.